Driver assistance systems are auxiliary devices in a vehicle that support the driver when driving the vehicle. Such driver assistance systems typically include various subsystems such as driver information systems or predictive safety systems. Some of these subsystems require an environmental sensor system that monitors the surrounding environment of the vehicle in order for example to detect objects that may present obstacles on the roadway.
Typical methods for environmental recognition in a vehicle make use of distance sensors, in particular ultrasound sensors, that, based on a transmission pulse echo method, measure the distance from objects in the environment surrounding the vehicle, and whose measurement data are used as a basis for a reaction that is to be generated by the driver assistance system. The quality of the provided measurement data therefore plays a decisive role in the provision of a driver assistance function. Thus, sensor degradations or environmental influences such as snow or ice may cause disturbances that result in the complete loss of function of the ultrasound sensors. The ultrasound sensors are then quasi-blind. In order to make diagnoses of the functioning of the ultrasound sensors that are as reliable as possible, various methods are known.
In the simplest case, the functioning of the ultrasound sensor is measured by measuring whether the sensor is detecting a signal at all. From DE 10 103 936 A1, an ultrasound sonar system is known for detecting an obstacle, in which an ultrasound oscillator produces a post-oscillation or decay oscillation frequency. By changing from a first transmission frequency to a second transmission frequency different from the post-oscillation frequency, the presence of an echo, and thus of an obstacle, within a specified distance can be inferred. However, such methods cannot provide a reliable assessment of whether the sensor is impaired or nonfunctional.
A second group of methods measures a transfer function at various excitation frequencies, and is capable of extracting information therefrom concerning the frequency characteristic and the signal quality. DE 10 2010 003 624 A1 describes a method for acquiring disturbances of an ultrasound transducer. The ultrasound transducer is excited with two different frequencies, and the resulting different release time durations are compared. However, the measurement, which is a function of frequency, requires a good deal of time, and during this time the sensor cannot be used for other purposes. In addition, additional electronics are required that make the configuration of the sensor more complex and more expensive.
The third group of methods makes use of the die-out characteristic of the sensor in order to monitor its functionality. From EP 0 816 865 A2, a method is discussed for self-testing a device for ultrasound runtime measurements, in which the die-out characteristic of the ultrasound sensor is evaluated after a transmission process. In “diagnostic” mode, the die-out characteristic is evaluated with regard to at least one signal shape parameter, and is monitored to check the observance of an error criterion. Here, the die-out characteristic can be measured with each send-receive cycle. However, it is difficult to make reliable assessments of the functionality of the sensor on the basis of the die-out characteristic.
A further disadvantage of the known methods is that a total failure of ultrasound sensors is difficult to recognize. In particular when used in driver assistance systems, such a total failure means that the system is at least partially “blind.” Therefore, there is an ongoing interest in reliably recognizing possible disturbances, in particular those that result in complete loss of functionality.